Using mix compile, not mix as makeprg for mix compiler

Author: kelvinst

compiler/mix.vim

@@ -7,5 +7,5 @@ if exists(":CompilerSet") != 2
     command -nargs=* CompilerSet setlocal <args>
 endif
 
+CompilerSet makeprg=mix\ compile
 CompilerSet errorformat=%A%t%*[^:]:\ %m,%C%f:%l:\ %m,%C%f:%l,%Z
-CompilerSet makeprg=mix